1. Field of the Invention
The present invention relates to a metal halide lamp, and more particularly to an electrode utilized in a metal halide lamp.
2. Related Background Art
A metal halide lamp is an electric discharge lamp in which metal halides are added to the contents of a sealed tube or a bulb, and a lamp which is capable of improving luminous efficiency and color rendering properties utilizing a spectral line of each added metal atoms or emission of molecular spectrum of each metal halide and, if desired, capable of obtaining emission in a certain wavelength region. These metal halide lamps are utilized as wide illumination for roads, sports fields etc., room lighting for stores, a light source for an overhead projector, a liquid crystal projector, a slide projector etc.
Conventionally, a cylindrical tungsten electrode having the same diameter along its entire length, a tungsten electrode in which a diameter of a proximal portion (portion near a sealed portion of a bulb) is large, which is disclosed in Japanese Patent Laid-Open No. HEI 5-82086 (82086/1993), and a tapered tungsten electrode in which a diameter gradually becomes small from a distal end or an inner end to a proximal portion, which is disclosed in Japanese Patent Laid-Open No. HEI 4-280057 (280057/1992) are known as tungsten electrodes used in general metal halide lamps.
In these conventional metal halide lamps, a part of an electrode near a sealed portion of a bulb reacts with a metal halide and generates a low-melting alloy. This low-melting alloy tends to deposit on a spherical portion or luminous portion of the bulb to whiten the luminous portion. Further, the deposited low-melting alloy may react with silica glass of which the bulb is made, thereby to occur crystallization, which may cause decrease of pressure resistance of a bulb and damage of a bulb.